The fourth man holds a fritillary and an ear of sweetcorn - plants which Griffiths says point to Shakespeare's poem Venus and Adonis and his play Titus Andronicus.

Below the bearded fourth man - who wears a laurel wreath - was "an ingenious cipher of the kind loved by the Elizabethan aristocracy" which, when decoded, confirmed his identity as "William Shakespeare".

Edward Wilson, emeritus fellow of Worcester College, Oxford, explained said that he and Griffiths had spent five years consulting Latin and Shakespeare scholars before going public.

"We do not think anyone is going to dispute this at all," he said.

Griffiths writes in Country Life: "The Fourth Man is not cartoonish or stylised. It may be monochrome, in fancy dress, and just 3.5 inches tall, but this is something that has been sought for centuries."

He goes on: "By the time that portraits of Shakespeare were at a premium, the significance of the Rogers engraving had faded from memory. Its camouflaged figures, coded plants and ciphers proved too clever for its own good.

"The title page, one of the richest and most important artworks of the English Renaissance, came to be seen merely as a bibliophile's rarity and a fine, if stereotypical specimen of Elizabethan decoration. Nobody dreamed of finding Shakespeare in it."